Catalytic converters for vehicle exhaust systems are well known. Such converters are typically constructed by having a housing within which one or more converter bricks (often called converter matrices) are arranged. Each of the bricks has a washcoat containing one or more rare metals typically chosen amongst the elements of platinum, rhodium or palladium.
The washcoat provides a plurality of catalytic reaction sites on which oxygen which is temporarily stored within the catalytic converter as a buffer (i.e. an amount of surplus oxygen for later use) during lean mixture running, can undergo catalytic oxidation reactions with one or all of the following gases: carbon monoxide (CO), hydrocarbons (HC) and nitrous oxides (NO.sub.x) of various types. Where a three-way catalytic converter is provided, all the gases undergo oxidation reactions.
It has been observed previously however that the oxygen buffer capacity of the catalytic converter diminishes with time. In turn, the conversion capabilities of the catalytic converter become steadily reduced such that the catalytic converter will be unable to cope with large fluctuations in the value of A (commonly adopted measure of air/fuel ratio). Such large fluctuations may occur for instance when the engine's fuel injection control unit alters the injection time so as to achieve stoichiometric combustion conditions in the engine during acceleration or deceleration.
It is known that by taking a reading of the outputs of the front and rear oxygen sensors (typically so-called "lambda" sensors) and processing these values, a depleted buffer capacity can be detected thus allowing a deteriorated catalytic converter to be replaced in good time.
One such method for determining the presence of a reduced buffer capacity of a catalytic converter is disclosed in e.g. DE-A-38 30 515, in which the difference in the oxygen content of the exhaust gas both upstream and downstream of the catalytic converter is measured. By comparing the quotient of this difference to the oxygen quantity upstream of the catalytic converter, a value is obtained which can be compared to known values. On the basis of the comparison, the condition of the catalytic converter is determined so that replacement of the catalytic converter at an appropriate time can be instigated.
In a further prior art device for determining catalytic deterioration as disclosed in U.S. Pat. No. 5,228,335, the oxygen content signals from an oxygen sensor upstream and an oxygen sensor downstream of the catalytic converter are fed into a microprocessor and compared to threshold values. On the basis of said comparison, the deterioration level is determined.
The aforementioned prior art methods thus rely on the detection of a specific set of conditions arising in order to be able to determine catalytic converter failure.
The present invention aims at providing an arrangement and method for determining the size of the oxygen buffer capacity of a catalytic converter, in particular at repeated intervals during engine operation so that updated information on buffer capacity is available.
In a further aspect of the invention, the value of the oxygen buffer capacity is used to provide a control input to the fuel injection control unit to alter the fuel injection to said engine in order to compensate for reduced buffer capacity. Any method of measuring oxygen buffer capacity can be used to provide the required information for such compensation.